U.S. Pat. No. 4,816,125 describes a MEMS microphone with a piezoelectric layer made from ZnO and several electrodes connected to this layer that are arranged concentrically.
The following publication describes a microphone module with an encapsulated MEMS microphone, in which an enclosed air volume (back volume) is in a housing underneath the microphone's diaphragm: J. J. Neumann, Jr. and K. J. Gabriel, “A fully integrated CMOS-MEMS audio microphone,” the 12th International Conference on Solid State Sensors, Actuators, and Microsystems, 2003 IEEE, pp. 230-233.
The following publication describes an electrical module with an installed MEMS piezoresistive microphone: D. P. Arnold, et al., “A directional acoustic array using silicon micromachined piezoresistive microphones,” J. Acoust. Soc. Am., Vol. 113(1), 2003, pp. 289-298.
The following publication describes a piezoelectric microphone, which has two piezoelectric layers made from ZnO and a floating electrode arranged in-between: Mang-Nian Niu and Eun Sok Kim, “Piezoelectric Bimorph Microphone Built on Micromachined Parylene Diaphragm,” Journal of Microelectromechanical Systems, Vol. 12, 2003 IEEE, pp. 892-898.